Method of making a wire support leadframe for a semiconductor device

ABSTRACT

A leadframe includes a plurality of interconnected support members. A pair of die pads is connected to the support members and configured to receive a pair of dies electrically connected by at least one wire. A support bracket extends between the die pads and includes a surface for maintaining the at least one wire at a predetermined distance from the die pads during overmolding of the leadframe.

TECHNICAL FIELD

The present invention relates generally to leadframes and, inparticular, relates to a wire support bracket for a leadframe.

BACKGROUND

Circuit boards include a leadframe and a series of electrical componentselectrically connected by wires. Once the components are assembled andconnected, an electrically insulating material is overmolded over thecircuit board to form the finished product and protect the componentstherein.

SUMMARY

In one example, a leadframe includes a plurality of interconnectedsupport members. A pair of die pads is connected to the support membersand configured to receive a pair of dies electrically connected by atleast one wire. A support bracket extends between the die pads andincludes a surface for maintaining the at least one wire at apredetermined distance from the die pads during overmolding of theleadframe.

In another example, a circuit board includes a leadframe having aplurality of interconnected support members a pair of die pads connectedto the support members. A support bracket extends between the die padsand includes a surface. A pair of dies is secured to the die pads. Atleast one wire electrically connects the dies together. The surface ofthe support bracket supports the at least one wire to maintain the atleast one wire at a predetermined distance from the die duringovermolding of the circuit board.

In another example, a method of forming a circuit board includes moldinga leadframe having a plurality of interconnected support members, a pairof die pads connected to the support members, and a support bracketextending between the die pads. The support bracket has a surface. Apair of dies is secured to the die pads. The dies are electricallyconnected with at least one wire such that the at least one wire extendsover the surface of the support bracket. The circuit board is overmoldedwith an electrically insulating material such that the surface of thesupport bracket spaces the at least one wire from the dies by apredetermined distance following molding.

Other objects and advantages and a fuller understanding of the inventionwill be had from the following detailed description and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a front isometric view of an example leadframe for acircuit board.

FIG. 2A illustrates the leadframe of FIG. 1 following a first stage ofmanufacturing.

FIG. 2B illustrates a top view of the leadframe of FIG. 2A.

FIG. 2C illustrates a front view of the leadframe of FIG. 2A.

FIG. 3 illustrates the leadframe of FIG. 1 following a second stage ofmanufacturing.

FIG. 4A illustrates the leadframe of FIG. 1 following third and fourstages of manufacturing.

FIG. 4B is a top view of FIG. 4A.

DETAILED DESCRIPTION

The present invention relates generally to leadframes and, inparticular, relates to a wire support bracket for a leadframe. FIGS. 1-4illustrate one example leadframe 20 for a circuit board, e.g., a smalleraligned integrated circuit (SOIC). Referring to FIG. 1, the leadframe 20extends generally in a plane from a first end 22 to a second end 24 andincludes a series of interconnected support members 26. The supportmembers 26 form a network having a plurality of spaces or gaps 28. Theleadframe 20 is formed from a plate of electrically conductive material,e.g., copper or copper-alloy, that is stamped and/or etched to definethe support members 26 and spaces 28 in a desired configuration.

A pair of die pads 32, 36 formed integrally with the support members 26are provided between the first and second ends 22, 24. The die pads 32,36 are illustrated as rectangular but could have any shape. The die pads32, 36 extend parallel to one another and are spaced apart by a passageor gap 54. Although two die pads 32, 36 are shown the leadframe 20 couldinclude more than two die pads.

First and second dies 50, 52 are secured to the pair of die pads 32, 36with an epoxy or the like. Each die 50, 52 constitutes a block ofsemiconductor material, e.g., electronic-grade silicon orgallium-arsenide, in which one or more functional circuits (not shown)is provided. The first die 50 includes a planar top surface 56 having aninner edge 57. The second die 52 includes a planar top surface 58 havingan inner edge 59. In one example, the dies 50, 52 are identical and thetop surfaces 56, 58 are co-planar. Alternatively, the dies 50, 52 aredifferent and/or the top surfaces 56, 58 extend parallel to one another.As shown, the first die 50 is a ball bonded die and the second die 52 isa stitch bonded die.

One or more electrically conductive wire or wire loops 60 extendsbetween the dies 50, 52 to electrically connect the dies to one another.As shown, a plurality of wires 60 extends between the dies 50, 52. Eachwire 60 includes a first end 62, a second end 64, and a middle portion66 extending from the first end to the second end. The first end 62 ofeach wire 60 is electrically connected to the first die 50. The secondend 64 of each wire 60 is electrically connected to the second die 52.The wires 60 are secured to the dies 50, 52 such that the wires 60extend substantially parallel to one another, with the middle portion 66of each wire extending over the gap 54 between the die pads 32, 36. Inone example, the wires 60 are gold wires, although other materials, suchas copper, can be used to form the wires.

A support bracket 80 extends between the die pads 32, 36 for supportingthe middle portions 66 of the wires 60 between the dies 50, 52. Thesupport bracket 80 extends along a centerline 81 from a first end 82 toa second end 84. At least one middle portion 86 extends between thefirst and second ends 82, 84. In one example, the middle portion 86extends parallel to the die pads 32, 36 and dies 50, 52. The first end82, second end 84, and middle portion 86 extend out of the plane of thesupport members 26 and die pads 32, 36.

The first end 82 of the support bracket 80 includes a pair of legs 90connected to the support members 26 and/or the die pads 32, 36 on oneside of the dies 50, 52 (the lower side as shown in FIG. 2A). An opening92 extends between the legs 90 to the gap 54. The legs 90 extend fromthe support members 26 towards one another and interconnect at thecenterline 81.

The second end 84 of the support bracket 80 includes a pair of legs 94connected to the support members 26 and/or the die pads 32, 36 on theother side of the dies 50, 52 (the upper side as shown in FIG. 2A). Anopening 96 extends between the legs 94 to the gap 54. The legs 94 extendfrom the support members 26 towards one another and interconnect at thecenterline 81. The legs 90, 94 can have any shape suitable forconnecting the support member 80 to the rest of the leadframe 20. Asshown, the legs 90 form a triangular first end 82 and the legs 92 form atriangular second end 84.

The middle portion 86 constitutes a single, planar member having a topsurface 88 extending along the centerline 81 and positionedequidistantly between the dies 50, 52. In another example, the middleportion 86 can constitute multiple planar members that extend parallelto one another and are evenly/unevenly spaced between the dies 50, 52(not shown). As shown in FIGS. 2A-2B, the middle portion 86 has apredetermined width W, e.g., on the order of about 4 mm. Where themiddle portion 86 constitutes multiple planar members, each middleportion will have the same width W. The middle portion 86 of the supportbracket 80 is coated with a solder resistant layer of, for example, apolymer, to prevent the wires 60 from shorting.

As shown in FIG. 2C, the top surface 88 of the middle portion 86 ispositioned a predetermined height H₁ from the top surface 34 of thefirst die pad 32. The top surface 88 is positioned a predeterminedheight H₂ from the top surface 38 of the second die pad 36. As shown,the heights H₁, H₂ are identical and can be, for example, about 630 μm,although the height(s) can be greater or less to meet design criterion.

During manufacturing, the leadframe 20, including the support members 26and support bracket 80, is formed first (FIGS. 2A-2C). The dies 50, 52are then secured via epoxy to the respective die pads 32, 36 (FIG. 3).Referring to FIGS. 1 and 4A, the first end 62 of each wire 60 iselectrically connected to the first die 50. The second end 64 of eachwire 60 is electrically connected to the second die 52. Each wire 60 canbe thermosonically bonded to both dies 50, 52. In one example, the firstends 62 are ball bonded to the first die 50 and the second ends 64 arestitch bonded to the second die 52. In any case, the wires 60 aresecured to the dies 50, 52 such that the middle portions 66 of the wires60 extend parallel to one another and over the planar surface 88 of themiddle portion 86 of the support bracket 80.

After the wires 60 are connected to the dies 50, 52 the entire leadframe20 is overmolded with an electrically insulating material, e.g.,plastic, in the direction indicated by the arrow A in FIGS. 1, 4A and 4Band illustrated in phantom by 110. During the overmolding process,however, the force of the mold material flowing over the wires 60 cancause plastic deformation of the wires from their initial positions. Forexample, the mold material can urge the middle portions 66 of the wires60 downwards towards the support members 26 in the direction D₁ (FIG.4A), also known as wire sagging. Alternatively or additionally, theflowing mold material can urge the middle portions 66 of the wires 60 tomove sideways in the direction D₂ (FIG. 4B) and/or sideways in thedirection D₃, both known as wire sweeping.

Since the ends 62, 64 of the wires 60 are rigidly fixed to the dies 50,52 any wire sagging or sweeping causes the wires to plastically deform.If the mold flowing forces are not counteracted and the wires 60 free tosag or sweep, the height of the wires from the edges 57, 59 of the dies50, 52 can be reduced to below a threshold amount. When this occurs,electrical arcing can occur between the wires 60 and dies 50, 52 duringuse of the circuit board, which can cause damage to the circuit boardand is therefore clearly undesirable.

As shown in FIGS. 4A-4B, the support bracket 80 advantageously supportsthe wires 60 during overmolding and helps ensure the middle portions 60are spaced a predetermined height/spacing from dies 32, 36 duringovermolding. In particular, the planar surface 88 of the middle portion66 is positioned at the height H₁ relative to the first die pad 32 (seeFIG. 2C) such that the first end 62 of the wire 60 maintains or staysabove a minimum/predetermined height H₃ (FIG. 4A) relative to the edge57 of the first die 50 during overmolding. Similarly, the planar surface88 of the middle portion 66 is positioned at the height H₂ relative tothe second die pad 36 (see FIG. 2C) such that the second end 62 of thewire 60 maintains or stays above a minimum/predetermined height H₄ (FIG.4A) relative to the edge 57 of the first die 50 during overmolding.

The heights H₃, H₄ can be identical or different from one anotherdepending on conditions such as the geometry of the wires 60,overmolding conditions, etc. In one example, the heights H₃, H₄ are atleast about 250 μm. In any case, since the dies 50, 52 are provided onthe die pads 32, 36 the support brackets 80 necessarily help ensure theends 62, 64 of the wires 60 are maintained at or stay abovepredetermined distances from the respective die pads during overmolding.

The support bracket 80 achieves proper support of the wires 60 byproviding an upward reaction force to the middle portions 66 of thewires 60 as the overmold urges the wires toward the leadframe 20 and/orsideways. To this end, the support bracket 80 is configured to have astiffness that is high enough to withstand external forces thereon thatcause wire sweep and wire sag during overmolding, thereby preventing themiddle portions 60 from plastically deforming during overmolding.

Moreover, frictional engagement between the middle portions 66 and theplanar surface 88 provides additional resistance to sideways movement ofthe wires 66 in the directions D₂, D₃. To this end, the solder resistantcoating on the surface 88 can increase this frictional resistance.Consequently, the support bracket 80 advantageously prevents the wires60 from sagging and sweeping to maintain the ends 62, 64 of the wires atthe predetermined heights H₃, H₄ throughout overmolding—or prevent theends 62, 64 from moving below the respective predetermined height. Thishelps prevent unwanted arcing during use of the circuit board andalleviates the need to x-ray inspect the overmolded leadframe for wiresag and wire sweep, thereby saving manufacturing time and cost.

Without the bracket 80, the first and second ends 62, 64 of the wires 60would need to extend higher relative to edges 57, 59 of the dies 50, 52to try and maintain the wires above the threshold height duringovermolding and account for wire sag and wire sweep. This construction,however, is unreliable, does not constrain the wires in any way orprovide reaction forces against the incoming mold flow, increasesmanufacturing, and material cost.

What have been described above are examples of the present invention. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the presentinvention, but one of ordinary skill in the art will recognize that manyfurther combinations and permutations of the present invention arepossible. Accordingly, the present invention is intended to embrace allsuch alterations, modifications and variations that fall within thespirit and scope of the appended claims.

What is claimed is:
 1. A method of forming a circuit comprising: forminga leadframe having a plurality of interconnected support members, a pairof die pads connected to the support members, and a support bracketextending between the die pads, the support bracket having an uppersurface; applying an insulating layer to the upper surface of thesupport bracket; securing a pair of dies to the respective die pads,each die having an upper surface; electrically connecting the dies withat least one wire such that the at least one wire extends over the uppersurface of the support bracket; and overmolding the leadframe with anelectrically insulating material, the upper surface of the supportbracket spacing a portion of the at least one wire above the uppersurfaces of the dies during overmolding.
 2. The method recited in claim1, wherein the support bracket spaces a middle portion of the at leastone wire above the upper surface of each die.
 3. The method recited inclaim 1, wherein the insulating layer comprises a polymer.
 4. The methodrecited in claim 1, wherein the insulating layer comprises a solderresistant layer.
 5. The method recited in claim 1, wherein the at leastone wire contacts the insulating layer.
 6. The method recited in claim1, wherein the step of forming comprises stamping the leadframe from aplate of electrically conductive material.
 7. The method recited inclaim 6, wherein the electrically conductive material comprises copper.8. A method, comprising: forming a leadframe having a plurality ofinterconnected support members; forming a plurality of die padsconnected to the support members, each die pad arranged to receive arespective die; and forming a support bracket having an upper surfaceand extending between the die pads, wherein the upper surface is formedabove an upper surface of each die.
 9. The method recited in claim 8,comprising applying an insulating layer to the upper surface of thesupport bracket.
 10. The method recited in claim 8, wherein theinsulating layer comprises a polymer.
 11. The method recited in claim 8,comprising securing a die to each respective die pad.
 12. The methodrecited in claim 11, comprising electrically connecting the dies with atleast one wire such that the at least one wire extends over the uppersurface of the support bracket.
 13. The method recited in claim 8,comprising overmolding the leadframe with an electrically insulatingmaterial.
 14. The method recited in claim 8, wherein the step of formingcomprises stamping the leadframe from a plate of electrically conductivematerial.
 15. The method recited in claim 14, wherein the electricallyconductive material comprises copper.
 16. A method of forming a circuit,comprising: forming a leadframe having a plurality of interconnectedsupport members; forming a plurality of die pads connected to thesupport members; securing a die to each respective die pad, each diehaving an upper surface; and forming a support bracket having an uppersurface, the support bracket extending between the die pads and abovethe upper surfaces of the dies.
 17. The method recited in claim 16,comprising applying an insulating layer to the upper surface of thesupport bracket.
 18. The method recited in claim 16, comprisingelectrically connecting the dies with at least one wire such that the atleast one wire extends over the upper surface of the support bracket.19. The method recited in claim 16, comprising overmolding the circuitwith an electrically insulating material.
 20. The method recited inclaim 16, wherein the step of forming comprises stamping the leadframefrom a plate of electrically conductive material.